This invention relates to flexible polyurethane foams used in shipping and packing cartons that isolate or reduce the effects from externally applied shocks or vibrations and thereby protect the contents of the carton.
Cellular polyurethane structures typically are prepared by generating a gas during polymerization of a liquid reaction mixture comprised of a polyester or polyether polyol, a polyisocyanate, a surfactant, catalyst and one or more blowing agents. The gas causes foaming of the reaction mixture to form the cellular structure. The surfactant stabilizes the structure.
Polyurethane foams with varying density and hardness may be formed. Hardness is typically measured as IFD (xe2x80x9cindentation force deflectionxe2x80x9d) or CFD (xe2x80x9ccompression force deflectionxe2x80x9d). Tensile strength, tear strength, compression set, air permeability, fatigue resistance, and energy absorbing characteristics may also be varied, as can many other properties. Specific foam characteristics depend upon the selection of the starting materials, the foaming process and conditions, and sometimes on the subsequent processing. Among other things, polyurethane foams are widely used as energy absorbing cushions and filler in the packaging industry.
Once the foam-forming ingredients are mixed together, it is known that the foam may be formed under either elevated or reduced controlled pressure conditions. PCT Published Patent Application WO 93/09934 discloses methods for continuously producing slabs of urethane polymers under controlled pressure conditions. The foam-forming mixture of polyol, polyisocyanate, blowing agent and other additives is introduced continuously onto a moving conveyor in an enclosure with two sub-chambers. The foaming takes place at controlled pressure. Reaction gases are exhausted from the enclosure as necessary to maintain the desired operating pressure. The two sub-chambers, a saw, and air tight doors are operated in a manner that allows for continuous production of slabstock polyurethane foam.
U.S. Pat. No. 4,777,186 to Stang, et al., describes a method of foaming in a pressurized chamber held above atmospheric pressure (i.e., in the range of about 0.5 to 1000 psig). In addition to the gases emitted during foaming, additional gases may be introduced into the chamber to maintain the elevated pressure during foaming. The resulting foams have a higher ILD to density ratio than those previously known in the art.
U.S. Pat. No. 6,034,148 to Kelly, et al., discloses energy absorbing foams formed with graft polyols, preferably low functionality, low molecular weight graft polyols with high solids content in the range of 35% to 55%. The foaming is carried out at controlled pressures above atmospheric pressure from about 1.2 to about 1.5 bar (absolute). The resulting foams have a density from 1.0 to 4.0 pounds per cubic foot, air permeability between 20 to 140 ft3/ft2/min and pre-flex CFD from 7 to 13 psi.
Energy absorbing polyurethane foams are also disclosed in U.S. Pat. No. 5,698,609 to Lockwood, et al. The open cell polyurethane foams are prepared from a combination of specific polyols reacted with diphenylmethane diisocyanate (MDI) and polyphenylmethylene diisocyanate (poly-MDI) at atmospheric pressure. The resulting foams have a density from 1.5 to 5 pounds per cubic foot and an air flow of 0.05 to 0.5 scfm.
Those of skill in the packaging industry characterize dynamic shock cushioning characteristics (xe2x80x9cenergy absorptionxe2x80x9d) of materials by developing xe2x80x9cdrop curvesxe2x80x9d or plots of deceleration versus static load in accord with ASTM 1596. A foam is cut to a predetermined size, typically 8xe2x80x3xc3x978xe2x80x3xc3x972xe2x80x3 (thickness) and positioned on an impact surface. A dropping platen with an adjustable load is dropped into the sample. Instrumentation measures both the peak impact deceleration (xe2x80x9cG-levelxe2x80x9d) and impact velocity as the platen deflects the cushion. The impact velocity is checked to be within tolerances, and the peak xe2x80x9cG-levelxe2x80x9d is recorded. The impact velocity corresponds to a xe2x80x9cfree fall drop height,xe2x80x9d which is measured in order to compensate for the effect of friction in the dropping apparatus, but the corresponding free fall drop height is typically reported as if it were measured physically. The most commonly used free fall drop height is 24 inches. The platen, with the same static loading, is dropped on the same cushion five times. The static loading is calculated by dividing the mass of the platen by the surface area of the foam sample. Each drop is separated by about one minute. A new cushion sample is used, and a sequence of five drops is performed for another static loading, usually determined by the experience of the operator during the test. The process is repeated until enough data points have been gathered to draw a representative curve. The average of the second through fifth drops in commonly reported as the average xe2x80x9cG-levelxe2x80x9d for each static loading.
Lower xe2x80x9cG-levelsxe2x80x9d indicate greater energy absorption by the foam, or less shock felt by the platen or what would be the packaged object in packaging applications. Prior art packaging materials with a density of about 2.2 pcf using a two-inch sample thickness and a 24-inch xe2x80x9cfree fall drop heightxe2x80x9d generally yield G-levels above 60 G at a 1 psi static loading. Prior art conventional foam packaging with a density of about 1.4 pcf yields G-levels above 70 G at a 0.4 psi static loading.
Articles more susceptible to damage have generally lower G values in the range of about 15 to 80 G. More rugged articles (such as television sets and VCRs) have G values generally in the range of about 80 to 100. Significantly rugged articles (such as furniture) may have G values exceeding 115.
An object of the present invention is to produce energy absorbing foam with xe2x80x9cdrop curvesxe2x80x9d substantially improved over those previously obtained in the prior art. Where foams with improved xe2x80x9cdrop curvesxe2x80x9d are used in packaging applications, either less foam material is required for the same energy absorbing protection, or the foam may be used to package heavier objects than previously possible. Ideally, a lighter foam with lower density that has better or at least equivalent drop curve performance will be used to reduce shipping weight and associated shipping cost.
According to the invention, foams with improved energy absorption are obtained by a method in which a foam-forming composition of:
(a) from 10 to 50% by weight total polyol of a polyether polyol having a functionality in the range from about 2.2 to 3.5 and a hydroxyl number in the range of about 28 to 168 and containing up to 30% EO;
(b) from 50 to 90% by weight total polyol of a graft polyol having a functionality in the range from about 2.5 to 3.0 and a hydroxyl number in the range of about 25 to 50 and containing from 50/50 to 80/20 of styrene/acrylonitrile;
(c) a polyisocyanate containing at least 5% by weight toluene diisocyanate and at least 80% by weight total polyol of methylene diisocyanate (MDI), wherein at least 50% by weight of the methylene diisocyanate is 4, 4xe2x80x2 methylene diisocyanate; and
(d) a blowing agent, is mixed together and foamed under controlled pressure conditions (vacuum) from about 0.5 to 0.9 bar, preferably 0.5 to 0.8 bar. Preferably, the isocyanate index is in the range of 95 to 110, most preferably 100 to 105.
Most preferably, the foam-forming composition contains up to 2 parts per hundred parts polyol of an amine catalyst, up to 2 parts per hundred parts polyol of a surfactant, up to 0.5 parts per hundred parts polyol of an organotin catalyst, from 2 to 6 parts per hundred parts polyol of water as the blowing agent, and up to 2 parts per hundred parts polyol, preferably from 0.3 to 0.5 parts, of DEOA. In addition, it has been found that from 15 to 25% by weight total polyols of polyether polyol (functionality 3.1 to 3.3) and 75 to 85% by weight total polyols of graft polyol (functionality 2.8 to 2.9) is a preferred combination for polyols. Moreover, the polyisocyanate most preferably contains from 8 to 11% by weight toluene diisocyanate and from 50 to 60% by weight 4, 4xe2x80x2 methylene diisocyanate.
Surprisingly, foams prepared according to this invention exhibit excellent energy absorbing characteristics over those of other shipping/packaging foams. The resulting foams have a density in the range of 0.7 to 1.3 pounds per cubic foot, an air permeability from 25 to 140 ft3/ft2/min, preferably 25 to 60 ft3/ft2/min, and IFD25 of about 40 to 140 lb., preferably 60 to 110 lb. The foams can be made suitable for packaging applications such as electronic equipment, where desired IFD25 for the packaging material varies from 30 to 100 lb. At equivalent density, the packaging foams according to the invention are able to sustain much higher static stresses.